1. Field of the Invention
The present invention relates to a method of producing an insulating material for coating the heater elements of indirectly heated cathodes.
2. Description of the Prior Art
The use of aluminum oxide as an insulating material for coating cathode heater elements is known. The aluminum oxide is applied to the filament in the form of a fine powder either mechanically with a spray nozzle or electrophoretically in a coating bath. It then forms a white, highly insulating layer. The white color of the layer of aluminum oxide presents a high thermal resistance since the transfer of heat is effected primarily by way of heat conduction and only slight heat radiation takes place. Prior attempts have been made to overcome this disadvantage of the aluminum oxide by coating the insulating layer with a second darker layer. Up to now it was not considered feasible to employ a dark coloring throughout because the desired heating effect could not be achieved while also maintaining both the desired electrical insulation property and long service lifetime. The present improved method is used for making materials for coating cathode heater elements such as described in copending U.S. Application Ser. No. 418,644, filed Nov. 23, 1973, now U.S. Pat. No. 3,895,249, and assigned to the same assignee as the instant application.